This invention relates to cementitious compositions and to accelerators for use herein, particularly for sprayed concrete.
The application of concrete to a substrate by spraying from a nozzle (commonly referred to as xe2x80x9cshotcretingxe2x80x9d) is a well-established technology, and is widely used in such applications as the lining of tunnels. It is important that the sprayed concrete set very rapidly on the substrate, and this is achieved by the addition to the concrete at the nozzle of an accelerator. These accelerators are quite different from those used with conventional concrete and have traditionally included such materials as alkali metal hydroxides, aluminates and silicates.
The highly alkaline nature of these materials has given handing problems. It also means that their use in confined spaces such as tunnels has led to very unpleasant working atmospheres. Recent attempts to avoid such materials have involved the use of aluminium compounds and typical examples may be found in European Patents 0 076 927, 0 775 097 and 0 742 179, Australian Patent 706917 and European Applications 0 812 812 and 0 946 451.
It has now been found that it is possible to prepare an accelerator for sprayed concrete by a simple process, which accelerator performs especially well. The invention therefore provides a method of preparing an accelerator for sprayed concrete consisting essentially of the steps of
(i) dissolving aluminium sulphate and aluminium hydroxide in water which optionally contains at least one amine dissolved therein, to give a clear solution; and
(ii) optionally adding at least one of at least one stabiliser and at least one defoaming agent;
the proportions of ingredients present being such that the final product contains from 3%-12% by weight of aluminium sulphate (measured as Al2O3), up to 30% by weight of amorphous aluminium hydroxide, up to 15% by weight amine, up to 3% by weight defoaming agent and up to 0.06 mol/kg. stabiliser, the stabiliser being selected from hydroxycarboxylic acids, phosphoric acids and non-alkaline salts of phosphoric acids.
The invention additionally provides an accelerator for use with sprayed concrete prepared by such a method.
The aluminium sulphate used may be any commercially-available material. Aluminium sulphates differ in their purity and constitution, the most common being so-called xe2x80x9c17%xe2x80x9d because it contains 17% of Al2O3. In practical terms, the weight percentage of 17% aluminium sulphate, Al2(SO4)3. 14.3 H2O, which should be used in the process according to the invention lies in the range of from 30% to 60%, preferably from 40%-48%.
The aluminium hydroxide may be any commercially-available amorphous aluminium hydroxide. Although all such aluminium hydroxides will give satisfactory results, it is generally true that the more recent the date of manufacture, the better the result. In addition, aluminium hydroxides which, as a result of their particular manner of manufacture, contain a small proportion of aluminium carbonate (up to 5%) are easier to dissolve and are preferred materials. This behaviour is not obtained by simply adding aluminium carbonate to pure aluminium hydroxide. Although very small quantities of aluminium hydroxide may be used (less than 0.1% is possible), a significant improvement is observed at 5% or more. The preferred range of weight proportions is from 8-25%, preferably from 15-25%.
Although aluminium sulphate, aluminium hydroxide and water can, when utilised together in the process of the invention, give accelerators with good properties, the properties can be considerably enhanced by the use of one or more of three optional, but preferred, components.
The first of these is amine. This must be water-soluble, otherwise there is no restriction on the choice of amine. Preferred amines are alkanolamines, such as diglycolamine, diethanolamine and triethanolamine, diethanolamine being particularly preferred. Up to 10% by weight amine may be used, preferably from 4-7%.
The second preferred additional component is stabiliser, which may be added at the end of the process. This is a material which prevents the aluminium hydroxide/aluminium sulphate solution either from precipitating or from forming a gel. Without stabiliser, the solution will function well as an accelerator, but it will often lack stability and therefore shelf life, necessitating its use very shortly after manufacture, something usually not practical. It is possible and permissible to use more than one stabiliser.
The stabilisers for use in this invention are hydroxycarboxylic acids, phosphoric acids and non-alkaline salts of phosphoric acids. The hydroxycarboxylic acid may be selected from any such acid known to the art. The preferred acid is citric acid, but many other acids, such as lactic acid and ascorbic acid may also be used.
By xe2x80x9cphosphoric acidxe2x80x9d is meant one of the acids orthophosphoric acid (H3PO4), metaphosphoric acid ((HPO3)x) and pyrophosphoric acid (H4P2O7). By xe2x80x9cnon-alkaline saltsxe2x80x9d is meant salts which do not include the alkali metals sodium and potassium. Thus, for example, lithium, calcium and magnesium phosphate salts may be used.
The third preferred additional component, defoaming agent, may be any such material known to the art. Most of these are proprietary commercial materials whose precise composition is never revealed, but any such material known to the art is suitable. Typical examples include silicone types such as AGITAN (trade mark) and fatty acid polyether types such as LUMITEN (trade mark) EL.
The defoaming agent may be used at a rate out up to 5% (solids by weight of the whole composition), preferably from 0.5%-3%. The use of defoaming agent makes the use of less fresh aluminium hydroxides easier. It is believed, without restricting the scope of the invention in any way, that its presence helps in the removal of carbon dioxide which accumulates on the surface of the aluminium hydroxide over time. Surprisingly, provided that the defoamer contains no silicone and that it is not present to the extent of more than 3%, it gives an appreciable improvement in setting time over that of an identical composition without defoaming agent or with silicone types.
The process of the invention is readily carried out with standard equipment, and the skilled person will have no difficulty in doing so. It will be appreciated that in order to achieve solutions at the various stages, some heating may be necessary, typically to about 50-60xc2x0 C.
In the process, the clear solution can be produced by any convenient method. It is possible to add the aluminium sulphate and aluminium hydroxide sequentially in any order to water. It is also possible to add them together to water, or to dissolve or disperse them individually in two different quantities of water and then combine these quantities.
Preferably, the aluminium sulphate and the aluminium hydroxide are added sequentially to water. Preferably the aluminium sulphate is first dissolved in water; aluminium sulphate will dissolve with heating. To this solution the aluminium hydroxide is then added. A clear solution is obtained.
It is possible, although less preferable, first to add the aluminium hydroxide to the water. Aluminium hydroxide does not dissolve readily in water, but gives a fine suspension. To this suspension the aluminium sulphate is added. A clear solution is obtained.
The precise nature of the product of the process is not known. It is certainly not a mere mixture of the original components (the fact that the product is a clear or slightly turbid solution and not an opaque suspension typical of aluminium hydroxide is evidence of this), and without restricting the invention in any way, it is believed to be oligomeric or polymeric in nature.
The accelerator thus prepared gives excellent results when used as a shotcrete accelerator. Shotcrete treated therewith hardens rapidly and has good final strength. The accelerator has a long shelf-life, is resistant to changes in temperature and is completely non-alkaline, thus leading to better working environments.
The invention is further illustrated by the following non-limiting examples.